1. Field of the Invention
The invention relates to a draw frame with a control means for compensating fluctuations in the mass of the sliver to be processed. The invention is particularly preferable in connection with draw frames in the called preparatory stage of a spinning mill, e.g. in drafting devices or combing machines.
2. Description of the Related
It has long been known, in particular in so-called autoleveller draw frame's, to compensate fluctuations the mass of a sliver (to be spun later on) by changing the draft in a controlled manner. A particularly preferable arrangement for carrying out such a process is shown in our Swiss Patent Application No. 2834/89 of Jul. 31, 1989. The transfer of this function to combing machine has been proposed in our European Patent Application No. 376 002.
In this context it is also known that the most difficult problems with respect to the process arise during the start-up and the braking of the draw frame. The importance of these start-up and braking problems are exacerbated by the continuously rising delivery speeds (with continuously rising productivity) of the respective machines. At a normal supply speed of 800 m/min the start-up and braking period for a draw frame is approx. 1 to 3 secs. If a defect sliver is produced because of problems in a control circuit within such a period, the defect can have an effect on the subsequent spinning of fine yarn up to a yarn length of approx. 700 to 2000 m.
The sliver treated in the draw frame has to be deposited in a so-called can for transportation between the processing stages. The draw frame usually has to be briefly turned off after the filling of a can for the purpose of changing the can, which requires a braking period and then a start-up period.
A control means for coping with the latter problem is the subject matter of our Swiss Patent Application No. 2537 of Jul. 13, 1990.
It is the task of a controlled draw frame, either in the drafting device of a combing machine or an other textile machine, to compensate to the utmost extent both long wave and short wave mass fluctuations of the sliver to be drawn. The compensation of long wave fluctuations does not cause any major problems, whereas the compensation of short wave fluctuations still does. In connection with the compensation of short wave mass fluctuations there are problems in respect of
the measurement of the fluctuations, PA1 the (highly dynamical) drive of the drafting rollers in accordance with the measurements, PA1 the transformation of the roller speed to the suitable linear movement of the slivers. PA1 in the feed zone a multiple doubling takes place, i.e. at least four and preferably six to eight slivers are introduced or means for introducing such a number of slivers are provided; PA1 the drafting device comprises both a predrafting zone as well as a main drafting zone; PA1 a driving means for the drafting device is provided; PA1 a (sliver) mass measuring unit is provided in the feed zone and a control means reacting thereto and acting upon the driving system in order to at least reduce mass fluctuations recognized by the measurement unit by way of changing the draft in the main drafting zone, whereby the supply speed (i.e. the delivery speed) is changed; PA1 the fleece supplied by the drafting device joined to a sliver or means for this purpose are provided; PA1 the sliver thus formed is intermediately stored in a storage means; PA1 a sensor is provided that reacts to the amount or the length of the intermediately stored sliver; PA1 a can press for depositing the sliver drawn from the storage means is provided, whereby the can press is integrated in the driving system; PA1 the said control means reacts to the said sensor and acts upon the driving system in such a way that either the feed speed is kept constant and the speed of withdrawing the stored sliver from the storage means by mean of the can press changes in a controlled manner in order to maintain the predefined sliver amount or sliver length in the storage means, or the feed speed is changed with the withdrawing speed from the storage means remaining constant, or both (feed and withdrawing speeds) are controlled in order to maintain the stored amount.
From patent literature it is known that the change in the draft that is required for the compensation can be gained by controlling the delivery speed in a drafting device with two drafting zones (see, for example, the DE-OS 1 685 627). This arrangement, however, has not yet gained ground in general practice. The feed speed is usually changed instead. When feeding the machine, however, from a plurality of spinning cans, this arrangement can lead to complications in the feeding zone of the machine.
It is also known to provide the carding machine with a drafting arrangement comprising a single drafting zone in order to compensate sliver unevenness "at the source" by controlling the draft at relatively low supply speeds. Examples of such arrangements can be found in the CH-PS 462 682, DE-OS 22 30 069DE-AS 19 31 929 and DE-AS 25 43 839. Such "supplementary carding units" also comprise a storage means that is able to smooth the high-frequency changes of the supply speed before the transfer to the next processing stage. This arrangement, too, has never been employed in general practice for longer periods.
A storage means has not become known in combination with a high-speed draw frame (such as, for example, a frame according to US-PS 4 413 378 or EP-62 185).
Our own European Patent Application No. 376 002 provides the use of a storage means in combination with the control in the feed of a drafting device of a combing machine, but not in combination with the control of the delivery.